Introduction
In seismic exploration seismic reflection data from subsurface layers are collected by multiple receivers typically at the earth's surface. The more recent 3D seismic acquisitions cover large areas, collecting extensive 3D seismic data for use in subsurface imaging.
Seismic “migration” corrects and improves initial assumptions of near horizontal layering in an attempt to model geophysical realities such as dips, discontinuities, and curvature of formations. Seismic migration is typically the culmination of the image processing for the seismic data, with the goal of producing detailed pictures for use in the interpretation of subsurface geologic structures. Such detailed pictures are important for prospect generation and reservoir characterization, such as lithology, fluid prediction, and pore pressure prediction, as well as for reservoir volume estimation.
3D seismic interpretation of migrated data aspires to produce an accurate mapping of the subsurface structures, important for oil and gas exploration. Such seismic interpretation profits from an improved focusing and positioning of subsurface reflectors in the migrated data. It is one disclosure of the instant invention that such improved focusing and positioning can be made possible by migrating the data with a more realistic velocity model.
Seismic migration of 3D prestack seismic data is practiced in two forms, time migration and depth migration. Each requires a 3D velocity model, a description of acoustic velocity structures. Time migration is simpler, less resource intensive and considered to be less accurate and less sophisticated. Historically, the industry has not been bothered to build a sophisticated geologically plausible velocity model prior to prestack time migration, relying instead on simple assumptions deemed to be fitting the technique regarded as less accurate. For accuracy, the industry currently looks to time and resource intensive “depth migration”, a technique utilizing massive interations to converge on solutions.
One disclosure of the instant invention is that, surprisingly, an effective and efficient focusing and positioning of subsurface reflectors is achievable by seismic time migration, but such results are highly dependant on the accuracy of the 3D velocity model employed therein. In particular, a geologically realistic, accurate 3D velocity model produces surprisingly good results when used with curved ray prestack time migration.
The relatively new 3D seismic depth imaging services, techniques that have opened a rapidly growing market in recent years, are at present highly interpretive, quite expensive and very complex processing jobs. They carry the promise of illuminating complicated oil traps under geologic complications, such as sub-sak prospects in deep water so that to economize on the high cost of drilling in deep water, such better seismic imaging technology is in demand. However, reducing the cost and time of such complex processing is therefore also important. One disclosure of the instant invention is a methodology that so reduces the cost and time.
Interpreters are historically given the task of tying well markers to 3D seismic data. Calibrating velocity models derived from seismic data and used for migration by tying into well information (hard data) has been historically deemed the interpreter's task, preformed post migration. Inconsistencies between processing velocities and the depth tying velocities, however, create formidable challenges at the interpretation stage. It is a significant improvement of the instant invention to have previously dealt with this calibration issue, prior to and/or during migration.
The instant invention provides a better solution, to both imaging and tying well information. More focused seismic data, produced as a result of early velocity calibration with hard data and geologic data and the use of geostatistically sensitive trend fitting (together referred to as “iDEPTHing”) prior to migration, as taught herein, restores better signal-to-noise ratio for deeper exploration. By utilizing the methods of the instant invention curved-ray prestack time migration provides excellent migrated seismic data quickly to interpreters, and provides superior preliminary data for constructing a 3D velocity model for prestack depth migration.
Summary of State of the Industry
The current industry practice of prestack depth migration is best described by John W. C. Sherwood (Sherwood, 1989); “How can we routinely, efficiently, and economically extract interval velocities with the accuracy need for depth inversion? . . . . First, an educational estimate of the interval velocities is made. This is used to create, from the regular stacked (may be migrated) section, an approximate depth-interval velocity model. This first guess is going to be wrong. But the data can be processed on the basis of this model using a full pre-stack depth migration. Then the data are examined and a determination is made of how well it is focused. If they don't appear to be focused properly, the model can be adjusted, both the depths and the interval velocities, to bring the data into better focus. The adjustment is repeated until the results are satisfactory. The procedure is today being used very intensively by some companies.” An abundance of articles on the subject of iterative prestack depth migration have been published for the last 20 years (Wang et. al., 1991; Lee and Zhang, 1992; MacKay and Abma, 1992; Liu, 1997).
U.S. Pat. No. 4,992,996, entitled “Interval velocity analysis and depth migration using common reflection point gathers” relates to a method of performing velocity analysis while eliminating the effects on weak signals caused by stronger signals with an assumption that assumes that the initial velocity model is reasonably correct and geologically plausible. The current invention provides a method for providing such a needed geologically plausible initial velocity model.
The iterative technique of depth migration often fails to converge, and it often fails to provide prospects with its economics within reasonable ranges of error for drilling. The iterative technique is based on the assumption that all iterations will converge to a unique solution most of the times. However, these iterations will converge to a geologically acceptable solution only when the initial model is geologically plausible. And, as Sherwood stated, the initial model is going to be wrong most of the times: the target depth is often wrong based on the result of iteration. Again, the current invention provides a method for providing such a needed geologically plausible initial velocity model.
Another favorite assumption is that imaging velocities are different from velocities for depth conversion due to anisotropy. It has been assumed that seismic velocities are in general 2-13% faster than the well average velocities (Guzman et. Al., 1997). However, considering errors in seismic velocity measurements 5-10%, anisotropy cannot be so easily assumed. Based on hyperbolic moveout, vertical velocity gradients and velocity trend reversal will be a significant source of errors. Small errorsin stacking velocities will be amplified in converting to interval velocities.
As a reference for geophysical solutions being non-unique based on geophysical data collected on earth surface, Al-Chalabi anabzed explicitly the non-uniqueness in velocity-depth curve in his article (Al-Chalabi 1997). He pointed out that the errors in measuring velocities from semblance clouds make the problem even worse. However, the current industry practice assumes the existence of a unique or true solution. The current invention deals with velocity calibration in an improved manner in order to reduce inaccuracies in seismic velocities and non-uniqueness.
U.S. Pat. No. 5,089,994, entitled “Tomographic estimation of seismic transmission velocities from constant offset depth migrations” relates to a method for improving velocity models so that constant-offset migrations estimate consistent positions for reflectors, including tomographic estimations of seismic transmission velocities from constant-offset depth migrations. This costly method has a fundamental problem of non-uniqueness between layer velocities and layer boundary positions. The current invention is a prerequisite to reasonable tomographic estimation.
U.S. Pat. No. 5,513,150, entitled “Method of determining 3-D acoustic velocities for seismic surveys” relates to a method of producing a velocity volume for a seismic survey volume, based on two-way time seismic data and process velocity data. This method provides a hand-on interactive tool for velocity editing but does not recognize the importance of velocity calibration.
Constructing improved velocity models requires all different sources of velocity data. Inaccurate seismic velocity (soft) data are abundant and reliable well data (hard) data are sparse. Most of the times, inconsistencies between seismic and well data are observed. Mostly the inconsistencies between different sources of velocity data are attributed to some unexplained physical mechanism. However, it is difficult to access any physical mechanism due to inaccuracies in seismic velocity measurements. The following patent stated that the inconsistencies might come from anisotropy.
U.S. Pat. No. 6,253,157, entitled “Method for efficient manual inversion of seismic velocity information”, relates to a method of calculating seismic velocity for migration purposes as a function of subsurface spatial position that gives the seismic processing analyst direct control of the resulting migration velocity model. This method recognizes the different sources of velocity information and the strengths of one source often offset the weaknesses of one another. It states that different velocity information sources usually give inconsistent estimates of the instantaneous velocity and interpreted that this inconsistency may be due to anisotropy that is being ignored.
Anisotropy is difficult to measure because it can only be measured in the laboratories. Laboratory measurements indicating that there were significant intrinsic anisotropy was observed in West Africa, and anisotropy migration is required for both improved imaging and accurate positioning in such cases.
U.S. Pat. Nos. 5,696,735 and 6,002,642, entitled “Seismic migration using offset checkshot data” relates to a method of migrating seismic data using offset checkshot survey measurements. This method uses direct travel time measurement from offset checkshot survey instead of a 3D velocity model. Although, such offset checkshot surveys are not commonly available, they may be advantageously used in special situations for shear wave imaging of dipping reflectors.
Development of Instant Invention
It was postulated that velocity models constructed according to current industry standards could be too smooth for depth imaging and sometimes wipe out key prospects. Excessive smoothing in velocity modeling could be a consequence of embedded velocity errors. Often layer boundaries were interpreted based on over-migrated or under-migrated results. It was detrimental to include erroneous boundaries in a velocity model.
A variation of the collocated Co-Kriging method was early on tested by the instant inventor on post-stack seismic data for modeling geologic features such as geo-pressure zones. The collocated Co-Kriging method showed that it could incorporate big local anomalies with smooth transition boundaries (Lee and Xu, 2000), integrating seismic (soft) data and well (hard) data by the zone of influence according to variances. Calibration was used in the test to tie wells for post-stack depth migration. Focusing or residual velocity or focusing errors could not be determined with post-stack data. The primary purpose of the post-stack depth migration was to correct positioning errors.
In a recent subsequent test, in accordance with the instant invention, velocity calibration to hard and geologic data and geologically sensitive trend fitting (together referred to as “iDEPTHing”) was incorporated in constructing a geologically plausible velocity model for preslack time/depth migration. The test showed that curved-ray prestack time migration based on such velocity model gave astonishingly (“eureka”) good results in terms of imaging steeply dipping seismic events and tying well markers (Kenney and Lee, 2003; Lee, 2003). Further, the output of such time migration provides a superior, cost effective and efficient input for any subsequent depth migration.
Current Invention
The current invention discloses improved methods for constructing a premigration velocity model by the preferred steps of: 1) editing to avoid embedding velocity errors and to improve lateral velocity trends of seismic velocities; 2) better fitting of vertical velocity trends by calibrating seismic (soft) data to well (hard) data (checkshot and/or sonic logs); 3) more accurately interpolating velocities and calibration scale factors using geostatistical techniques; 4) incorporating geologic features, such as known stratigraphic horizons, into a stratigraphic grid, when their positioning accuracy is verified, and otherwise utilizing known geologic features for editing; and 5) avoiding excessive smoothing by using a variation of geostatistical collocated co-Kriging for localized anomolies.
The invention teaches that curved-ray prestack time migration will correctly accommodate a vertical velocity gradient. Further, the prestack curved ray time migration results using the instant invention provide superior focusing of seismic events, including steeply dipping events, and excellent well ties. Due to well ties, more accurate stratigraphic horizons can be interpreted. A second velocity calibration (to hard data and geologic data) and (geologically sensitive) trend fitting (together referred to as “iDEPTHing”) before subsequent prestack depth migration can significantly accommodate lateral velocity variations above a prospect. For sub-salt prospects, the inventive methods can provide valuable calibrated velocity data in above top salt for constructing a geologically plausible initial model for prestack depth migration.
Geostatistical methods have been used in other disciple, such as reservoir characterization. Interpreters have used velocity calibration. Some predecessors have attempted to calibrate well (hard) and seismic (soft) data prior to migration. The current invention, however, is the first to specifically disclose a unique (referred to as “iDEPTHing” process for calibration prior to prestack time and/or depth migration as well as its surprising value when combined with curved-ray prestack time migration, and when then subsequently used with depth migration.